Nowadays, it is strongly required to reduce the weight of a car body from the viewpoint of improving fuel efficiency of an automobile. In addition to that, increasing the collision safety of a vehicle is also required from the viewpoint of occupant protection in a collision of vehicles. An increase in the strength and a decrease in the thickness of car body parts are being realized by the application of a high strength steel sheet to the parts in order to achieve both a decrease in the weight and an increase in the collision safety of a car body at the same time, as described above.
However, since strengthening of a steel sheet is accompanied by a decrease in press formability in terms of, for example, ductility or deep drawability, it is difficult to apply a high strength steel sheet to press-formed parts for which a high formability is required. For example, a steel sheet having a low tensile strength of from 270 MPa to 340 MPa and excellent formability has been used for automobile outer panels such as a door outer panel and a back door panel. It is necessary to maintain high formability in order to increase the strength of these parts. Total elongation observed in a tensile test is used as one of the indicators of formability. The larger the total elongation which is the sum of uniform elongation and local elongation, the more intensive the work to which a workpiece can be subjected until the workpiece is broken. However, in the case where the deformation of a steel sheet becomes larger than uniform elongation and reaches the range of local elongation in a practical press forming process, the deformation is concentrated at a portion at which local elongation occurs, which results in necking in which a decrease in thickness is concentrated at the portion. Therefore, the portion at which necking occurs can be easily broken and raises the danger of fracture in a press forming process. Moreover, there is the deterioration of the surface appearance of an outer panel in the case where necking occurs at the design surface of the panel. For these reasons, high uniform elongation is required of a steel sheet to be used for an outer panel. In addition, up to now, it has been also strongly required to suppress the occurrence of yield point elongation of this kind of steel sheet in order to prevent the deterioration of surface appearance quality due to the occurrence of stretcher strain.
At the same time, the application of a galvanized steel sheet to car body parts is being developed, because it is important to secure the corrosion resistance of car body parts from the viewpoint of prolonging the service life of car bodies. Therefore, zinc coatability also has become one of the important requirements of a high strength steel sheet.
A Dual-Phase (DP) steel sheet, in which a hard martensite phase is dispersed in a soft ferrite phase, is well known as one of the approaches to increase the tensile strength and ductility of a steel sheet at the same time. A DP steel sheet is excellent in terms of shape fixability due to having a low yield point as well as a high strength, and, moreover, exhibits comparatively high uniform elongation due to having an excellent work hardening property (refer to, for example, Patent Literature 1). However, it is difficult to say that a DP steel sheet has sufficient formability equivalent to that of a kind of steel sheet having strength of from 270 MPa to 340 MPa.
Therefore, a TRIP steel sheet, which utilizes a transformation-induced plasticity (Transformation-induced Plasticity: TRIP) effect which exhibits high ductility, in particular, high uniform elongation by having formed a retained austenite phase in a soft ferrite phase and by using the transformation of the austenite phase into a martensite phase when deformation occurs, is well known as an approach to achieve better ductility. Among TRIP steel sheets, a Si added TRIP steel sheet, in which the formation of carbides is delayed by adding Si, then concentration of C in an austenite phase is promoted and an austenite phase is stabilized, is well known (refer to, for example, Patent Literature 2). However, although a Si added TRIP steel sheet has an excellent balance of TS×EL, the steel sheet has low ductility and a small absolute value of uniform elongation, because the steel sheet inevitably has a tensile strength of 590 MPa or more when the steel sheet contains an enough amount of Si to secure the retained austenite phase, since Si is a chemical element which has a very strong effect of solid-solution strengthening. In fact, a conventional cold-rolled Si added TRIP steel sheet has a uniform elongation of less than 24%. Moreover, Si tends to form a strong oxidized film on the surface of a steel sheet in the processes of slab heating, hot rolling and annealing, which results in the occurrence of surface defects such as residual scale, uneven plating and bare spots. Therefore, a Si added TRIP steel sheet needs improvement from the viewpoint of surface appearance quality also.
Therefore, an Al added TRIP steel sheet is proposed, since Al promotes concentration of C in an austenite phase as Si does, and compared with Si, Al has a smaller effect of increasing strength and decreasing zinc coatability. For example, Patent Literature 3 discloses a method for manufacturing a steel sheet having excellent ductility and adhesion of coating, which has a certain amount of retained austenite phase produced by reducing a content of Si and keeping an Al content of from 1.5% to 2%, with a tensile strength (TS) of from 440 MPa to 490 MPa and a elongation (El) of from 36% to 39%. However, in order to produce this steel sheet, it is essential to distribute Mn beforehand by performing annealing under conditions for a dual phase, that is, at a temperature of 800° C. for a duration of about one hour prior to final annealing, and, moreover, in a process of annealing and plating, it is necessary that the steel sheet be cooled after undergoing soaking annealing of 800° C.×60 seconds and undergo annealing of a long duration of 10 minutes or less after undergoing dipping in a plating bath at a temperature of 440° C. That is to say, in this method, it is difficult to manufacture the steel sheet in a common CGL line which does not have an annealing line following a hot-dip plating line, and, moreover, there is a problem of a large increase in production cost, because it is necessary to perform a heat treatment prior to final annealing. Moreover, as described in the examples, since any of the preferred steel sheets according to the above inventions have yield point elongation of more than 2%, there is the deterioration of surface appearance quality due to the occurrence of stretcher strain when press forming is performed.
Although Patent Literature 4 also discloses a method for manufacturing galvanized steel sheet having high ductility in which a retained austenite phase is utilized effectively by decreasing a Si content and by adding Al, there is necessity of annealing at a temperature of 750° C. or more before final annealing in a CGL and tempering at a temperature of from 250° C. to 550° C.
Patent Literature 5 proposes a method for manufacturing a galvanized steel sheet which utilizes a cold-rolled steel sheet as a base metal containing Si and high contents of Mn and Al, having a tensile strength of from 440 MPa to 490 MPa and excellent ductility. However, there is a problem from the viewpoint of surface appearance quality in that the powdering property of the plating of the steel sheet disclosed in this literature is significantly poor, because the alloying temperature of the steel sheet is much higher than that in existing methods. In addition, since rapid heating is necessary, there is a significant increase in manufacturing cost in comparison to conventional CGL equipment.
Patent Literature 6 proposes a method for manufacturing a high Al containing steel sheet having an excellent TS×EL balance, other than the method in which steel having a high content of Al undergoes a heat treatment in advance of plating in a CGL, in which plating in a CGL is performed after forming a hot-rolled microstructure containing the volume fraction of a low-temperature transformation phase of 10% or more which contains the volume fraction of a bainite phase of 80% or more by coiling a hot-rolled steel sheet at a low temperature of from 350° C. to 500° C., without performing the heat treatment in advance of plating in a CGL. However, it is difficult to say that a sufficiently good zinc coatability can be achieved by using this method, because a chemical composition containing a certain amount of Si is adopted.
Patent Literature 7 discloses a method for manufacturing a high Al containing steel sheet exhibiting high EL. However, cooling ability in terms of a secondary cooling rate of 80° C./s or more is substantially essential for this method for manufacturing a steel sheet. Incidentally, the present inventors conducted investigations in a range of low cooling rate and found that the properties of some of the steel sheets according to the examples of the literature became significantly poor, because the second phase degraded into a pearlite phase.